i say analysis, but it may be rudins fault, since the analysis profs mostly use his book, and it is one of the least user friendly of all maths books. i.e. analysts are among the few remaining profs who take pride in presenting the material in a way that pleases themselves and not the student.
but i think pivoxa expressed my intrinsic reason perfectly about the subject getting smaller and tighter exactly where the action is. after years of practice, it seems the soloution is to stop short, accept that fact, and just be content that for every epsilon, all the rest of the action takes place inside an epsilon disc one has no need to imagine the center of.
for me topology was eaisest, algebra next, and analysis hardest. somehow algebraic topology is considered hard too, but the differential version of topology is much easier. to get into algebraic or differential topology without tears, anything by andrew wallace is recommended.
so again maybe it depends on the presentation. milnor is also great. for analysis i like dieudonne, even though he tries purposely to make it harder by drawing absolutely no pictures. but maybe his intent was to force the reader to draw them.
riesz nagy is also excellent but old fashioned. but then i have given all these recommendations in my mathematician thread, which of course is now too long for anyone to read.